<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
    <ul id="statuses"></ul>
    <script type="text/javascript">
        const list = document.getElementById("statuses");

        function appendOnlineStatus(onLine) {
            const label = onLine ? "device is now online" : "device is now offline";
            const txt = document.createTextNode(label);
            const li = document.createElement("li");
            li.appendChild(txt);
            list.appendChild(li);
        }

        window.addEventListener('offline', () => appendOnlineStatus(false));
        window.addEventListener('online', () => appendOnlineStatus(true));

        appendOnlineStatus(navigator.onLine);

    </script>
</body>
</html>
